In WebKit, the argument is already optional, so the bug title is confusing. Is the part in parentheses what you are actually changing here?
Unlike for Mozilla, defaulting to true changes behavior to exact opposite of what we had. Can the spec be fixed to match long-standing WebKit behavior instead?

100% compliant seems dubious given that the argument was only recently made optional in the specification. That WebKit would simply use undefined as an implicit default everywhere was unfortunate, but given that nobody else did that I don't think we should have automatically accepted that as precedent as it's pretty bad API-wise.

Anne, I think that you are talking about implementation details here. Observable behavior is that behavior of cloneNode() in WebKit was and remains compliant to DOM3, and DOM4 was written to specify something that did not match any browser.

Whether your throw or not for omitted non-optional arguments is not an implementation detail. Before Web IDL bindings were badly defined, but that does not mean that how WebKit decided to the bindings needs to be the standard. If you think it should be, I hope you understand this is the wrong forum.